The invention relates in general to welding and more particularly to joining together by welding plates or wall segments each comprised of one metal clad with another metal.
Some chemical reactions require high pressures and temperatures, and hence must occur within a pressure vessel, that is, within a vessel capable of withstanding the elevated pressures. The vessel must also resist attack by the chemicals which are introduced into it as well as those produced in the chemical reactions. While steel possesses substantial strength, it is not in and of itself ideally suited for the construction of pressure vessels since it reacts with quite a few substances. Certainly, it cannot be exposed to highly corrosive substances, such as many acids--and some highly corrosive substances are produced under elevated pressures and temperatures. Refractory metals, such as tantalum, zirconium and titanium, do not react as readily with most corrosive substances, even at high temperatures and pressures, and further possess considerable strength, but they are also quite expensive, and to manufacture a large pressure vessel from any one of them would be prohibitive. The industry has therefore turned to steel plates clad with refractory metal. Fabricators weld these plates together to create vessels in which the refractory metal claddings are exposed to the interiors of the vessels where the reactions occur. The claddings protect the steel from the corrosive substances in the vessels, whereas the steel withstands the stresses produced by the elevated pressures.
Refractory metals do not bond easily to steel. Indeed, much of the clad plate is derived from an explosive bonding process. In essence, a thin layer of refractory metal, which is covered with an explosive, is laid over a steel plate or substrate separated by small Styrafoam spacers. When the explosive is ignited, the explosion ejects the spacers and drives the refractory metal against the steel plate, producing a diffusion bond between the two. Where tantalum is the refractory metal, a thin layer of copper is normally interposed between the tantalum layer and the steel substrate. The copper bonds well to both the steel and the tantalum and further, being quite ductile, yields when the clad plate is rolled or otherwise formed into the contours required for pressure vessel walls. More importantly, it serves as a heat sink when the tantalum is welded and thus dissipates the heat so that the concentrated high temperature of the weld is not transmitted to the steel. In this regard, tantalum melts at 5200.degree. F., whereas steel melts at about 2800.degree. F. Were the tantalum directly against the steel, any weld in the thin layer of tantalum might melt the underlying steel and change the physical characteristics of the steel, thereby weakening the steel in that region. The intervening copper layer, on the other hand, absorbs much of the heat and distributes it so that the underlying steel does not experience excessively high temperatures and undergo a change in its physical characteristics.
Normally the tantalum layer has a thickness of no more than about 0.035-0.040 inches. At that thickness it bonds well to the copper, the explosive bonding procedure, producing a bond which is uniform throughout. However, some specifications call for a thicker cladding of tantalum--indeed a tantalum layer that is 0.060 inches thick or thicker. At that thickness, the explosive bonding procedure may not yield a uniform bond, but instead the bond may be characterized by a multitude of small domes or embossments in the tantalum layer. These domes or embossments represent regions where gas and ash is trapped between the tantalum and the copper, gas and ash which derives from the consumption of the Styrofoam spacers used in the explosive bonding procedure. At the embossments no bond exists between the copper and tantalum layers, and indeed the two layers are actually separated. The absence of a bond at these locations may not impair the operation of the pressure vessel, since the tantalum still isolates the steel from the interior of the vessel. But it does affect the fabrication of the vessel.
In this regard, the domes or embossments are numerous enough that at least several of them will lie along welds later made in the tantalum layer as part of the fabrication procedure for the vessel. Of course, the tantalum melts at the head of a narrow bead as the tantalum is welded, and when this bead reaches one of the embossments, it opens the pocket of trapped gas and ash at the embossment. Molten copper and tantalum spew from the pocket, leaving the tantalum in this region contaminated with copper. As such, the cladding in this region will not resist corrosive substances nearly as well.
Of course, one could cover the regions of the tantalum that are contaminated with copper with tantalum cover plates, but this is expensive and gives the appearance of poor workmanship.
The present invention resides in a process for joining clad plates having pockets of trapped gases between the cladding and substrate. It also resides in a pressure vessel or other fabrication formed from the plates joined by the welding process.